I'm doing my project on energy based auditing, it's an industrial project... I need to measure the high voltage of around 240v, can I step down to 5v using voltage divider?

  • \$\begingroup\$ not safely, no. you need a transformer for isolation. also remember that naive resistor dividers have polarity, unless you merely half the voltage. \$\endgroup\$ – dandavis Oct 24 '17 at 5:19
  • \$\begingroup\$ Yeah I know, but I was thinking of measurement without transformer,.... Is there any other methods for transformer less measurement of AC voltage? \$\endgroup\$ – Dexter Oct 24 '17 at 5:21
  • \$\begingroup\$ Yes: optical. So with an optocoupler. Note that this might not work over a very large AC voltage range and some circuitry is needed to get a proper indication of the AC voltage. \$\endgroup\$ – Bimpelrekkie Oct 24 '17 at 5:28
  • \$\begingroup\$ Are you wanting "instantaneous" measurement? Or are you willing to accept the peaks or some kind of mean value? Also, what is your allowable power budget for the device? \$\endgroup\$ – jonk Oct 24 '17 at 6:56
  • \$\begingroup\$ I need to measure the voltage using Microcontroller, so as you said need to measure at each samples and get the mean, if I use the divider what resistor values should i use for high voltage measurement? \$\endgroup\$ – Dexter Oct 24 '17 at 13:09

You can't accurately measure energy flow without measuring the instantaneous voltage and instantaneous current with many samples per cycle.

I suggest you take a look at reference designs for energy meters. If it's an industrial situation you may have to do it for 3-phase power. The safest and most expensive method is via a potential transformer.

  • \$\begingroup\$ I did the circuit using voltage divider and stepped down 230v to 5v ac, then i connected to my arduino to test to measure actual ac voltage by taking about 100 samples per second and got those values ;) \$\endgroup\$ – Dexter Jan 26 '18 at 6:15

Yes you can. But things become complicated quickly since there are a few other things to consider.

When you design such circuit you'd want to have as much resistance as possible to limit wasted power in the divider. You'd also want your divider to survive electrical fast transients (EFT) and other surges, thus your divider should be able to withstand those voltages.

But high total resistance makes common mode and noise pickup a problem. Yet low total resistance make dissipated heat a problem.
You could take 20 resistors to combat this, but then you'd also have the side effects of 20 resistors. (eg: space, inaccuracy and drift)

There several application notes around tackling this issue.
TI: slaa494a
Atmel: AVR465

Then you also have the problem of data acquisition, since you'd need to be doing RMS with enough bit depth and sample-rate. Which is quite cpu intensive.
And then the harmonics show up and your measurement is incorrect...

Notice that whenever you attach anything to live or neutral without high resistance or isolation you can't directly attach anything else anymore. No programmer without isolation, no USB, no oscilloscope.
Magic smoke will be released if you do.

  • \$\begingroup\$ Can i use anti alias filter to reduce noise? \$\endgroup\$ – Dexter Oct 24 '17 at 13:06
  • \$\begingroup\$ @Dexter Alias filter is mandatory anytime you have Analog to Digital conversion. It won't help with all the other problems though. \$\endgroup\$ – Jeroen3 Oct 24 '17 at 15:13
  • \$\begingroup\$ Very good advice about scopes, USB etc. I remember doing this kind of thing on a three phase motor design. I was so careful to disconnect everything after programming and before powering back up with rectified mains present while waiting for my isolation transformer to arrive. Then one time, inevitably, I forgot. My poor motherboard, may it rest in peace. Not to mention the JTAG interface that is now in the great electronics scrap heap in the sky. \$\endgroup\$ – DiBosco Jun 12 '18 at 15:48

Your cheapest solution would be a two stage step down transformer with a buffer in between; most commercial transformers are highly efficient and would save you the further trouble of worrying about noise.

  • 1
    \$\begingroup\$ Welcome to EE.SE. (1) Why would you recommend a two-stage step down? (2) How would two stages be cheaper than one? (3) What is a buffer between two transformers? (4) Why would efficiency matter on a voltage measurement application where the current is likely to be < 1 mA or so? (5) Why would noise be an issue with any other solution and not with a two stage step down? There is a schematic button on the editor toolbar if you wish to add a schematic of your answer. \$\endgroup\$ – Transistor Oct 24 '17 at 9:50
  • \$\begingroup\$ (1) I recommended two stages to ensure that the analog voltage would almost always be at bare minimum irrespective of spikes, and having a large step down transformer wouldn't be aesthetically pleasing for a small project, so two small step down transformers should do....(2) it wouldn't be cheaper..(3) i was thinking an op-amp buffer to ensure something of a pure voltage source with very small impedance..(4) I assumed that the OP gave preference to correctness, considering that his ADC resolution might be fantastic..(5) Commercial transformers have better Noise control for it's price \$\endgroup\$ – Igbokwe Danny Oct 24 '17 at 11:03
  • \$\begingroup\$ I appreciate your thought, but for my project i needed to reduce the bulky of my device, so I thought of transformer less high voltage measurement using microcontroller. Using Voltage divider we can achieve step down to around 5v range, but it's kinda dangerous anyway, if I do using divider, i think we can reduce noise by ANTI ALIASING FILTER which attenuate High frequency component, using this crosstalk and noise could be reduce i think. \$\endgroup\$ – Dexter Oct 24 '17 at 13:05

Just a thought but since your goal seems to be measuring energy rather than voltage, why don't you concentrate on measuring current and just assume the voltage is its nominal value. You didn't specify accuracy but mains voltage rather varies more than about 5% either above or below nominal. This would eliminate a difficult, and potentially dangerous, measurement process. Measuring current is not necessarily easy but it seems you must do it since energy is much more dependent on current rather than voltage.


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